As an electric motor for an electric vehicle such as electric car and hybrid car, a permanent magnet synchronous electric motor which has a high torque density and can be relatively easily made compact and can cause a high output is used in many situations. The permanent magnet synchronous electric motor used as an electric motor for the electric vehicle has such a tendency as to increase heat density due to compact size with the output maintained to thereby increase temperature. When the temperature of the permanent magnet is more than or equal to a predetermined temperature, the permanent magnet synchronous electric motor causes an irreversible demagnetization to thereby significantly decrease the output, therefore, for preventing such output decrease, monitoring of the temperature of the permanent magnet is needed. However, since the permanent magnet is disposed on the rotor side, directly measuring the permanent magnet by means of a temperature sensor is of difficulty. Thus, a method of estimating the temperature of the permanent magnet from other information is being considered.
As one of such estimating methods, for example, Patent Literature 1 describes a technology which makes a map of a relation between the winding wire temperature and the magnet temperature and estimates the magnet temperature by referring to a detection value of a winding wire temperature sensor.